Thin film transistor liquid crystal display (TFT-LCD) requires gamma voltage generator to generate gamma voltages corresponding to a gamma curve related to the characteristics of the TFT-LCD to adjust its display effect. Specifically, the gamma curve is typically symmetric in the manner that it has a central gamma voltage and two groups of gamma voltages symmetric to each other with the central gamma voltage as the symmetric center thereof. FIG. 1 shows a conventional gamma voltage generator 10, which comprises a voltage divider 12 connected between a supply voltage VS and ground GND, and the voltage divider 12 is composed of several resistors R1, R2, R3, . . . , Rk+1 connected in series, so as to divide the supply voltage VS to be several voltages VR1, VR2, VR3, . . . , VRk that are further buffered by respective operational amplifiers AMP1, AMP2, AMP3, . . . , AMPk to output the gamma voltages VG1, VG2, VG3, . . . , VGk. Since the gamma voltage generator 10 generates the gamma voltages by the voltage divider 12 composed of several resistors connected in series, whenever any one among these resistors in the voltage divider 12 is adjusted to tune the corresponding gamma voltage, all the other gamma voltages are also altered in the same time. In order to keep the other gamma voltages correct, any tuning among these gamma voltages requires the overall change of the resistors, and which is time-consuming and inconvenient in use.
To improve the above disadvantage, another gamma voltage generator 20 is proposed, as shown in FIG. 2, in which the gamma voltages VG1, VG2, VG3, . . . , VGk are generated from a supply voltage VS divided by resistor pairs [R10, R12], [R20, R22], [R30, R32], . . . , [Rk0, Rk2], respectively. When the gamma voltage generator 20 is desired to be adjusted with any one of the gamma voltages VG1, VG2, VG3, . . . , VGk, only the corresponding resistor pair is changed. Even though the gamma voltage generator 20 can be adjusted with its gamma voltages individually, the number of the resistors that are external to the chip they are connected is twice of that required by the gamma voltage generator 10, and as a result, the circuit of the gamma voltage generator 20 becomes more complex. Moreover, the chip using such gamma voltage generators is required to prepare more pins for the generated gamma voltages.
Therefore, it is desired a gamma voltage generator that requires less pins when it is used and is able to individually tune the gamma voltages it generates.